Mg(NO.sub.3).sub.2.6H.sub.2 O/MgCl.sub.2.6H.sub.2 O eutectic undergoes a phase transition at about 59.degree. C. (about 138.degree. F.) releasing or alternately absorbing a substantial quantity of latent heat of fusion. As such, the material has advantageous properties for use as an encapsulated heat storage medium for use in conjunction with solar energy systems for space or water heating applications. The material is particularly highlighted for such use by Yoneda and Takanashi "EUTECTIC MIXTURES FOR SOLAR HEAT STORAGE", Solar Energy, Vol. 21, pp. 61-63, 1978, herein fully incorporated by reference. In order to improve the material for such applications, however, there is a need to develop additives or a series of additives to modify the supercooling properties of hydrated Mg(NO.sub.3).sub.2 /MgCl.sub.2 compositions, and most advantageously, additives which are highly effective in a very small amount.